August 16, 2015

Normal and natural errors

ONE OF THE THINGS that small-boat
navigation teaches you is that nothing is precise. The best navigators make
allowances for the unknown factors that always affect small boats, especially
sailboats. They plot their positions within a circle of uncertainty and if
they’re seeking landfall at a particular spot on a coastline, they aim way off
to one side or the other, so they know which way to turn when they sight land.

When I was a lot younger I thought I
knew how to navigate with precision. This misconception was confirmed when I
sailed a 17-foot dinghy across the English Channel from Dover to Calais. I
studied the tide tables and figured out the speed and direction of the tidal
stream (or the set and drift as I used to call it then) for every hour. I then
drew my course on the chart and adjusted the compass heading to account for the
distance the tide was pushing me sideways in each hour. And thus, with a great
sense of triumph, I arrived off the rather featureless French coast exactly at
Calais.

It was beginner’s luck, of course.
Nobody can forecast the exact speed of the current, or its exact direction.
Nobody can tell you how much leeway your boat will make. Nobody can forecast
your exact speed or distance covered during any one-hour period, and so the
detailed markings you make so carefully on the chart turn out to be impractical
nonsense. In my case, it was probably a matter of all the errors canceling each
other out — a minor miracle in other words.

Years later, when further experience
had taught me some humility, I read The
Yacht Navigator’s Handbook, by Norman Dahl.[1] In the introduction he
says: “When I was first taught navigation (in the Royal Navy) errors were
thought of as being rather disgraceful, the sole result of poor technique by
the navigator. Whilst I always accepted (and still accept today) that I was not
the most brilliant navigator in the world, I was disappointed to find that,
however hard I tried, errors never seemed to go away. Navigating a submerged
submarine, and later, yachts of many kinds in many situations, eventually made
me realize that errors are an integral part of navigation and need to be
studied in their own right.”

Dahl said a major purpose of his
book was to show that errors in navigation are normal and natural, and that a
major skill in navigation lies in your ability to interpret the results in
terms of the likely errors. He goes on to show boat navigators how they can
actually use the errors to help make sensible decisions about their positions
and a future course of action.

As one who had never experienced any
difficulty in making errors I found Dahl’s advice very comforting, and I never
again tried to do anything as impossibly precise as maintaining a rhumb line
from Dover to Calais.

I expect Dahl’s book is out of print
now because it was first published in 1983, before the great revolution in
navigating that finally did bring near-precision to position-finding. We don’t
think of errors now, because GPS doesn’t allow for that. It will tell us our
position to within a boatlength in any kind of visibility, day or night.

And yet people have run aground
using GPS, often because GPS is more accurate than the charts you plot your
position on. There have been many reports of yachts wrecked on rocks, reefs,
and islands that were where either GPS or the charts said they weren’t.

So we now all find ourselves in the
position that I was in all those years ago, when I knew precisely and without
doubt how to cross the English Channel. It’s surely time we started doubting
again. It’s time we listened to Mr. Dahl, time we started taking all the
possible errors into account. Time to accept that navigation is never precise,
even with GPS.